In the 3rd Generation Partnership Project (3GPP), which is a mobile communication system standardization project, approaches for sophisticating Long Term Evolution (LTE) are promoted for keeping up with rapidly-increasing traffic demand. As one of such approaches, it is considered that not only a frequency band in which a license is granted to an operator (licensed band), but also a frequency band not requiring a license (unlicensed band) is used for LTE communication.
In the unlicensed band, a listen-before-talk (LBT) procedure is required for avoiding interference with another system (wireless local area network (LAN), etc.) being different from an LTE system, or with an LTE system of another operator. The LBT procedure is a procedure of monitoring a frequency in an unlicensed band, and thereby checking whether the frequency is vacant, based on reception power (interference power), and using the frequency only if it is checked that the frequency is vacant (clear channel).